Events on Thursday, October 16th, 2014

Abstract:
Non-Abelian anyons are widely sought for the exotic fundamental
physics they harbour as well as for their possible applications for
quantum information processing. Currently, there are numerous
blueprints for stabilizing the simplest type of non-Abelian anyon, a
Majorana zero energy mode bound to a vortex or a domain wall. One such
candidate system, a so-called "Majorana wire" can be made by
judiciously interfacing readily available materials; the experimental
evidence for the viability of this approach is presently emerging.
Following this idea, we introduce a device fabricated from
conventional fractional quantum Hall states, s-wave superconductors
and insulators with strong spin-orbit coupling. Similarly to a
Majorana wire, the ends of our “quantum wire” would bind
"parafermions", exotic non-Abelian anyons which can be viewed as
fractionalized Majorana zero modes.
I will briefly discuss their properties and describe how such
parafermions can be used to construct new and potentially useful
circuit elements which include current and voltage mirrors,
transistors for fractional charge currents and "flux capacitors".

Abstract: Part 1: The extragalactic gamma-ray background (EGB) is generated by the superposition of all extragalactic gamma-ray emissions and therefore provides a window on both the demographics and evolution of non-thermal phenomena across cosmic time. A significant fraction of the total EGB intensity has now been resolved into individual sources using the Fermi LAT, and there is an emerging understanding of how fainter members of the established extragalactic gamma-ray source classes can account for a majority of the residual approximately isotropic component of the gamma-ray sky, called the isotropic gamma-ray background (IGRB). The latest measurement of the IGRB spectrum with the Fermi LAT from 100 MeV to 820 GeV exhibits a high-energy cutoff feature consistent with the attenuation of high-energy gamma rays by pair-production on the IR/optical/UV extragalactic background light. High-energy cosmic neutrinos will be essential to see beyond this gamma-ray horizon to greater distances and higher energies.

Part 2: Targeted searches for indirect dark matter signals in the direction of Milky Way satellite galaxies provide some of the strongest current constraints on the annihilation cross section of dark matter derived from gamma-ray observations. Milky Way satellite galaxies have the advantages of low astrophysical backgrounds, the ability to constrain the dark matter abundance and distribution from the kinematics of member stars, and the opportunity to combine observations of multiple satellites in a joint-likelihood framework to enable more sensitive analyses. Accordingly, the discovery of additional Milky Way satellites in wide-field optical imaging surveys may provide substantial advances for indirect dark matter searches. I will discuss a matched-filter maximum-likelihood algorithm to search for and characterize ultra-faint galaxies in the ongoing Dark Energy Survey, which will cover 5000 square degrees in the relatively less explored south Galactic cap.

Abstract: As the nearest metal-rich, star-forming galaxy to the Milky Way, M31 plays a key role in understanding the interstellar medium (ISM) and star formation at z~0. Because of its proximity, we can study the properties of the ISM on the scale of individual star-forming molecular clouds and characterize the stellar sources of energy input for the ISM gas and dust. Recent observations have emphasized the importance cloud-scale processes in setting the efficiency of star formation. In particular, the formation of bound molecular clouds out of the cold neutral medium is a critical step in the process. Small spatial scale processes are also crucial for generating the tracers of star formation we use to study distant galaxies. M31 is currently the only metal-rich galaxy where existing observational facilities let us probe these important scales in all of the relevant tracers. Towards that end, we have assembled a powerful multi-wavelength observational dataset for a large portion of the disk of M31 - including HI 21-cm and radio continuum mapping from the VLA, CO J=(1-0) mapping from CARMA, Spitzer and Herschel photometry, Herschel spectroscopy, optical integral field spectroscopy, and resolved stellar photometry from the Pan-chromatic Hubble Andromeda Treasury (PHAT). I will summarize what we have learned about the connection between star formation and the ISM from this unique dataset.<br>